1. Field of the Invention
The general field of the invention is the area of fiber optics. In this case fiber optics are being used a sensing devices, more particularly, for remote sensing. Specifically, multi-mode fibers, which are sensitive to exterior phenomena, are used in conjunction with single mode fibers, which are insensitive to exterior phenomena to detect and analyze multiple phenomena remote locations.
2. Description of Related Art
Miller et al, U.S. Pat. No. 4,694,243 discloses a sensor capable of measuring physical quantities such as pressure, acceleration, and electrical currents by passing polarized and unpolarized light through an optical sensor which modifies the light in proportion to the magnitude of the physical quantities being measured.
Yoshihiro Ohtsuka Satoshi Tanaka in a May 1990 article in the Fiber and Integrated Optics, Volume 9, pp 245-253, discloses a Tension Induced Modal Birefringence in a Single Fiber Interferometric Strain Gauge.
Bohnert et al in Optics Letters, Vol. 17, No. 9, May 1, 1992 discloses an Interrogation of a remote elliptical core dual mode fiber strain sensor by using a tandem interferometer configuration to do remote sensing of strain.
Gerdt, U.S. Pat. No. 5,026,984 discloses methods for sensing temperature, pressure and liquid level by fiber optics in fuel tanks where there is a spark hazard present.
Kersey et al, U.S. Pat. No. 4,932,783 discloses methods for minimizing the polarization induced signal fading in an interferometric fiber optic sensor by interrogating the sensor using an input polarization modulated source.
Olsen U.S. Pat. No. 4,743,752 discloses a fiber optic remote sensor using fiber optics to link the signals or to provide power to remote electromechanical sensors.
There are a number of patents which disclose methods for measuring physical parameters by using fiber optic sensors such as: Henderson et al U.S. Pat. No. 5,034,679; Wickersheim et al U.S. Pat. No. 5,183,338; Wickersheim et al U.S. Pat. No. 5,112,137; Gillespie U.S. Pat. No. 5,168,538; Levy U.S. Pat. No. 5,158,720; and evans U.S. Pat. No. 3,590,374.
There have been disclosures which describe the use of single mode polarization preserving fibers which base the detection of perturbations on the distinct effect these perturbations have on each of the slow and fast polarization axes. These devices are limited in the amount of information they are capable of gathering and thereby limited on the number of parameters they can detect. This limitation restricts the detail with which a region can be inspected.
There have also been disclosures of devices which utilize two-mode and single mode fibers in combination to provide a sensing region which is sensitive to phenomena within in that region. These devices do use single mode fiber to insulate the information gathered within the sensing region from phenomena which occur outside of the sensing region. These devices do not disclose the use of polarized light as an element or restriction to the type of light transmitted through the fibers. Again, this type of device is limited to the amount of information it is capable of gathering and thereby the number of parameters it is capable of detecting.
A last area of related art is that disclosed by Kim et al, U.S. Pat. No. 4,915,468 (1990). Kim et al. discloses the general notion of a device which utilizes both polarized light and multimode fibers. This device is capable of sensing multiple parameters and thereby gathering more data than previous sensing devices. Kim et al. does not however teach one methods or devices for sensing in remote locations utilizing multiple parameter techniques. The invention of the instant application improves upon the general device disclosed by Kim et al. by providing a device which is capable of performing multiple parameter sensing in remote locations.